System of magnetron frequency modulation



Oct. 22, 1957 R. R. REED ETAL 2,810,

SYSTEM OF MAGNETRON FREQUENCY MODULATION Filed Sept. 21,1953

ATTORNEYS United States Patent MAGNETRON FREQUENCY MODULATION Application September 21, 1953, Serial No. 382,018

Claims. (Cl. BIS-5.13)

SYSTEM 6F This invention relates to an electron discharge tube of the type adapted to generate ultra-high frequency oscillations, generally referred to as magnetrons, and more particularly to one in which the frequency of one or more resonant circuits within the tube determines the frequency of the oscillations produced.

Tuning of the magnetron to an intended precise or narrow band frequency after the magnetron is entirely assembled, on test or in use is highly desirable. It is likewise desirable to be able to tune a magnetron While in use, from one precise frequency to another.

The present invention relates to and has for its object the tuning of a magnetron by electronic means, and more definitely, tuning by injecting electrons, the velocities of which are controlled by the accelerator voltage, into the capacitive elements of a magnetron, thereby altering the resonant frequency of the operating element or elements. According to the instant invention, this is done by placing an auxiliary cathode outside the oscillating elements of the magnetron anode, and applying a modulating voltage to a grid which controls emission from said auxiliary cathode. The electrons from the auxiliary cathode will be drawn toward the anode, their velocity being controlled by the accelerator voltage and will enter the oscillating cavities and if the frequency is somewhat less than the Larmor frequency, the electrons will have small radial amplitude due to the very strong magnetic field applied for magnetron operation and will travel in tight helices, the general directions of which are roughly straight paths. I propose using the presence of these electrons between the capacitive elements of the inherent resonant circuits to cause the desired change in the resonant frequency of the magnetron due to alterations of the dielectric constant and thereby obtain a frequency shift or tuning.

It is to be observed that the advantage of grid control of this disclosure is that the accelerator voltage controls electron velocity whereas, grid voltage controls only the beam current. The constant beam velocity is maintained during the modulation cycle giving greater Af (frequency shift) than with an ordinary diode type gun.

Other objects of the invention will appear to those skilled in the art to which it appertains as the description proceeds, both by direct recitation thereof and by implication from the context.

Referring to the accompanying drawing in which like numerals indicate similar parts throughout the several views:

Fig. l is an axial section through a magnetron showing my improved tuning means therein;

Fig. 2 is a view taken along line 22 of the Fig. 1 illustrating the configuration of the anode structure;

Fig. 3 is a detail view taken along line 33 of Fig. 1 illustrating the structure of the control grid;

Fig. 4 is a view taken along line 4-4 of Fig. 1 showing the underside of the auxiliary cathode and the electron emissive sources thereon.

In the specific embodiment of the invention illustrated in the several figures of the drawing, the reference nu- Patented Oct. 22, 1957 meral 10 designates a cylindrical metallic magnetron body, the ends of which have cover or end plates 11 sealed thereon. Within and as an integral part of said body is the usual magnetron anode structure 12 of generally cylindrical shape but shorter than the outer part of the body so as to provide end spaces 13 between the anode and said plates 11. The anode structure is axially hollow to provide a'cathode cavity and radiating from this cathode cavity are a plurality of cavities 14 constituting cavity resonators. Said cavity resonators 14 extend the full length of the anode body so as to open at their ends into said end spaces 13. The ends of the cathode cavity likewise open into the end spaces 13. Pole pieces 30 are sealed in axial apertures provided therefor in end pieces 11. Pole piece 39, on the left of the drawing, Fig. 1, can be moved in and out of the magnetron body 10, thereby varying the pole piece gap 13 on the left portion of Fig. l of the drawings.

A cathode 16 passes axially through the cathode cavity, adequately spaced from the anode and supported as usual by lead-in rods 17 entering the end cavities through apertures in the pole pieces. An output lead 18 is situated in one of the resonant cavities 14, passing out through the side wall of the body 10 in accordance with usual practice.

In carrying out the present invention, an auxiliary cathode 19 of circular channel shape is shown concentrically situated in one of said end spaces 13. The said auxiliary cathode occupies a position overlying the ends of the resonant cavities 14.

The said auxiliary cathode is shown as a channel, opening toward the end plate with the base facing the anode. Filament 24 is coiled within the channel and connected to an external source of excitation, not shown, in conventional fashion. On the underface of the auxiliary cathode, at areas directly in line with the ends of resonant cavities 14, are electron emissive sources 29. These sources may be of any appropriate material for copious emission of electrons of which alkaline earths are examples.

A control grid 31 of substantially washer shape is located between the anode and auxiliary cathode, coaxial with the latter. The control grid overlies both the auxiliary cathode and cavity resonators 14. Apertures 32, of a number equal to the number of electron emissive sources on the auxiliary cathode are provided on the control grid. The diameters of the said apertures are slightly less than the width of the corresponding portions of cavity resonators 14 and the electron emissive sources 29. A potential is applied to the grid from an external source, not shown, by conventional means.

Located between the grid 31 and the anode 12 is an electrostatic field shield 33 which is grounded as shown at 34. This shield prevents overlapping electric fields from causing the electrons from the cathode 16 to miss their anode target.

By virtue of the focusing of the magnetic field and the fact that the restricted emissive areas and grid apertures are directly over the slots of the anode, emissions are beamed into said slots with negligible spreading of the beams or scattering of the electrons. The potential applied to the accelerator controls the velocity of the electrons emitted by the auxiliary cathode while the grid voltage controls the beam current, changes in the latter controlling the frequency of the magnetron. The electrons will be collected at the far end of the anode block. Presence in and passage through said cavity resonators of electrons, alters the capacitance thereof and thus the capacitance of the circuit is directly aifected and the frequency shift will be proportional to the electron current, and can be controlled externally by the modulation power supply. Attention is called to the fact that while electrons are emitted from the main cathode 16 in a direction radially of the anode body, the electrons from the auxiliary cathode 19 are emitted axially of the anode body.

Having thus described an embodiment thereof, what I claim as my inventionis: a

1. A magnetron comprising a cylindrical anode, a cathode cavity located on the axis of said anode, a series of cavity resonators evenly spaced about'the cathode cavity,

a main cathode in saidcathode cavity, a circular channel shaped auxiliary cathode coaxial with andtspaced from said anode, electron emissive material on the surface of the auxiliary cathode facing the anode, a control-grid located between and coaxial with said main and auxiliary cathodes, said control grid having a plurality of circumferentially spaced apertures therein, eachin axial alignment with a cavity resonator, each of said apertureshaving a diameter less than the width of the portion of the cavity resonator with which it is in axial alignment, and a groundedelectrostatic field shield located between the auxiliary cathode and anode coaxial with said anode, main cathode, control grid,-and auxiliary cathode.

2. A magnetron comprising a cylindrical anode, a cathode cavity'on the axis of said anode, a series of cavity resonators evenly spaced about the cathode cavity, a main cathode in said cathode cavity, a circular channel shaped auxiliary cathode coaxial with and spaced from said anode, electron emissive material on the surface of' the auxiliary cathode facing the anode, and a control grid located between and coaxial with said main and anxiliary'cathodes,

said control grid having a plurality of circum'ferentially spaced apertures therein, each in axial alignment with a cavity resonator, each of said apertures having a diameter less than the portion of the cavity resonator with which it is in axial alignment. 7 3. In a rising-sun type magnetron having a main cath- V ode and an' anode with a series of evenly spaced cavity resonators, an auxiliary cathode having electron emissive areas corresponding in number to and directly aligned respectively with the anode cavity resonators, and a control grid located between the anode and said auxiliary cathode and having apertures opposite said electron emissive areas a cylindrical anode, a cathode cavity located on the axis of said anode, a series of cavity resonators evenly spaced about the cathode cavity, a main cathode for emitting electrons in said cathode cavity, an output lead inone of the cavity resonators, a circular channel shaped auxiliary cathode coaxial with and spaced from said anode,

electron emissive areas of a number corresponding'to the number of cavity resonators on the auxiliary cathode in axial alignment with said cavity resonators, a washershaped control grid coaxial with the auxiliary cathode and located between the anode and auxiliary cathode, said control grid having apertures of a number equal to said number of electron emissive areas and juxtaposed thereto, each of said apertures having a diameter less than the width of the portion of the cavity resonator with 'whichit is in alignment, and a grounded electrostatic field shield located between'the control grid and anode and,

coaxial with said anode, control grid, and auxiliary cathode for preventing overlapping electric fields from causing .the electrons from the main cathode to miss their anode 1 target.

References Cited in the file of this patent UNITED STATES PATENTS V Shoupp Apr. 19, 1949 2,467,538 2,468,243 Spencer 1 Apr. 26, 1949 2,508,473 Shoupp a May 23, 1950 2,538,597 Steele .Jan. 16, 1951 Cuccia Feb. 20, 1951 

